Correlated secondary TRC calibration method

ABSTRACT

This invention is a method of producing a set of TRC&#39;s for a color printer&#39;s secondary halftone screens that is correlated with the printer&#39;s primary halftone screens. The method makes use of the printer/screen characteristic data that is normally gathered during screen calibration. However, instead of progressing from the data to a normal calibration for the secondary screens, the method goes backward through the data starting with the finished primary screen TRC&#39;s. The method insures that for every primary screen density, the closest possible secondary screen density is used when the same digital value is specified.

BACKGROUND OF THE INVENTION

A method of avoiding artifacts when printing at the border between twodifferent halftone screens by using a computation to match the two tonereproduction curves (TRC).

In a printer, and more particularly in a color printer, each colorseparation is calibrated by generating a TRC which determines how muchtoner will be applied for a given image data input, over the entirerange of luminance. In this discussion, “luminance” is meant to coverdensity, Delta-e from paper, brightness or any other light measurement.A typical method of generating a TRC is to measurer the luminance of aset of color patches produced by the target printer and toner. These areplotted against the actual number of ON pixels in the dot to create asmooth “primary screen characteristic”. This curve is then normalized,transposed and rounded up or down to form the TRC, which is the samedata, but converted into individual points of integer values for use ina halftone screen. For a numerical example, in a system where the screenvalues are 0 to 256, if a patch that appears to be 50% gray is required,the digital output value may be quite a bit higher or lower than themathematical midpoint of 128 in order to put down exactly that amount oftoner to appear to be 50% gray. In this case the number “128” would bethe TRC input value, and a number somewhat different would be theoutput, to be used as the number of ON pixels in the screen. Several TRCcurves are generated, one for each separation, usually black, cyan,magenta and yellow.

It is also typical in color printing to have halftone screens of varyingfrequencies, with the higher frequencies being used for graphics andtext where accurate outlines are needed, and lower frequencies beingused for color pictures where a greater variation of colors is needed.Halftone screens that vary in some characteristic other than frequencymay also be used. In this discussion, frequency will be used as theexample. In this case, a TRC for each frequency of each separation mustbe generated. Then, as the raster proceeds from one type of screen toanother, the TRC's are switched from one set to another.

This system works well enough in an ordinary printer, but for qualitycolor printing, and especially if trapping is used, an artifact may beproduced at the border between screens of the same color but ofdifferent frequencies if the TRC's do not track together closely enough.For a numerical example, let us assume that the high frequency halftoneis 8×8 pixels with image data values of 0 to 64 and the low frequencyhalftone is 16 by 16 with values from 0 to 256, so that there is a ratioof 1 to 4. Therefore a high frequency image density value of 10 shouldbe the same color density as a low frequency image density value of 40.Further, the TRC's are not continuous lines but are actually a series ofpoints that are defined as integers, to be used to in a halftone screen,and so there is an inevitable amount of rounding up or down for eachvalue. If, for example, the point is rounded up in one TRC and down inthe other, there will be a visual artifact in luminance as the screen isswitched between them.

SUMMARY OF THE INVENTION

This artifact can be eliminated if the primary (low frequency) andsecondary (high frequency) curves track as closely as possible. This canbe accomplished by using each point on the primary screen TRC (ratherthan the secondary screen characteristic as in the prior art) as thestarting point to generate the corresponding point on the secondaryscreen TRC. This results in better tracking between the two because, tooversimplify, if the primary value was calculated by rounding up (ratherthan rounding down), and the secondary value is a function of theprimary, then the process will tend to result in a larger value for thesecondary as well, and better agreement results.

Points on the secondary TRC are generated from a series of equivalentpoints of the primary. The specific process for generating one point onthe secondary TRC from the corresponding point on the primary TRC can beexplained as follows, assuming that the secondary screen is n times thefrequency of the primary.

-   -   1. A point P2 on the primary TRC is selected.    -   2. A point P3 on the primary characteristic having the same        output value as P2 is selected.    -   3. A point P4 on the secondary characteristic having the same        luminance as P3 is selected.    -   4. A point P5 having 1/n² the input value of P2 and the same        output value as P4 is selected.    -   5. The point of the secondary TRC can now be determined by        rounding the output value of P5 up or down to the nearest        integer.

The two resultant TRC's will track more closely than they would have ifformed independently since, in this case, the secondary value is afunction of the rounding, up or down, of the primary.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical description of the relationships between theprimary and secondary screen characteristics and TRC's.

FIG. 2 is a flow chart showing the steps from page description languageto printed page.

FIG. 3 shows the halftone step 26 of FIG. 2 in more detail.

FIG. 4 is a diagram showing the situation in which the effect of thisinvention can be most clearly seen.

DETAILED DESCRIPTION OF THE INVENTION

In a typical system, a primary lower-frequency and more stable halftonescreen is used for pictorial information while a secondaryhigher-frequency screen is used for the edges of text, and also possiblyfor anti-aliasing and trapping.

Theoretically, independent calibrations of different halftone screensfor a printer should end up producing the same ink destiny from each ofthe screens when the same digital value is specified. And, on theaverage, this is what happens. However, on a level by level scale, thereis commonly enough difference in the calibrations that the human eye cansometimes see a transition from one screen to the other even when thesame value is specified.

These differences are partly due to the printer not being repeatablefrom page to page or from side to side on the same page in the densityit creates during calibration.

It is also partly due to noise in the densiometric or colorimetricmeasurements. A third source of error is the integer arithmetic that isused to create a TRC, there are only 256 possible values, and one screenmay switch to the next level at a slightly different value than theother.

The two different halftone screens require frequency calibrations alsobecause they may respond differently to drift on the xerographic setpoint. This drift could be in any or all the xerographic parameters suchas voltages, screen end-points, d-MAX, tribo conditions, humidity, etc.,that are normally controlled by internal feed-back circuits ormax-setups. While the internal feed-back can return to a nominalperformance for the primary screen, the new parameter settings might beless likely to return the secondary screens to their nominal conditionas they are more sensitive and have less latitude.

Because the engine response is typically different for different linescreens, and also because of considerable noise in measurements,independent calibrations of the two screens do not track together. Thiscauses problems when the screen is switched for edges.

Instead, this correlated calibration method uses a normal calibrationfor the primary screen to get the primary TRCs, but then goes backwardsthrough the measurement plots to achieve a set of TRCs for the secondaryscreen that produce the same density as the primary. The resultingsecondary TRC is not as smooth as a normal calibration would produce,but the secondary screen is not normally used in a situation like agradation or sweep where that would be a problem. As part of thecorrelated calibration, two adjacent sweeps with the two screens areprinted out to ensure that it is difficult to see the transition betweenthem.

FIG. 1 shows a Jones diagram that demonstrates the correlated TRCconcept.

In the normal Primary calibration, color patches are printed andmeasured to produce the Primary Screen Characteristic data for cyan,magenta, yellow, and black that is shown in the upper left quadrant ofthe plot. It is shown here as Delta-e from paper, but it might also beLuminance or even density data. From this data, with appropriatenormalization and other color information, the Primary Screen TRC's areproduced, as shown in the upper right quadrant.

In prior art calibration, this process is repeated independently formthe secondary screens.

In this correlated TRC method, the Secondary Screen Characteristic datais first produced as before, now shown in the lower left quadrant of thefigure. Then the new processing proceeds in a counter-clockwise fashionaround the Jones diagram from each color and each input value point.

A typical calculation for a cyan point starts at point P1 in thediagram. This value is passed through the Primary Screen TRC to get theTRC value P2. The value P2 is then passed through the Primary ScreenCharacteristic data by means of linear interpolation or a spline-fitroutine to produce the point P3. P3 is the Delta-e from paper that theinput value P1 will produce.

Then point P3 is passed backwards through the Secondary ScreenCharacteristic data, again by linear interpolation or spline-fit, toproduce point P4. Point P4 is the output value that produces theidentical Delta-e for both screens. This point P4 is then associatedwith Point P1 as one point P5 in the new Secondary Screen TRC. In thenormal course of events, P5 will not line up at an integer TRC value andwill have to be rounded up or down to the nearest integer. The completedSecondary Screen TRC is shown in the lower right quadrant.

The same process is repeated for the remaining color screens.

The maximum vertical and horizontal axis values for the primary andsecondary curves of the Jones diagram may frequently be different. Inthe FIG. 1 example they are 256 and 64 for primary and secondary screenTRC's, but a Jones diagram is routinely normalized so that both are thesame vertically and horizontally on the graph. In this latter case, forexample, the input values of P2 and P5 will be equal.

FIG. 2 is a flow chart of the entire prior art printing process,starting with a document 20 described in one of the page descriptionlanguages, where text is characterter coded and scanned in picturescould be in the form of multiple bits per image pixel.

These are rendered into printer-independent full color bit map raster 22at step 21.

Step 23 converts the image to the color space of the particular printer,using three dimensional characterization data 25, which reflects thepermanent or long term characteristics of the printer, to createdocument 24. This step includes color correction and undercolor removal.

Step 26 halftones the image pixels using calibration data 27 which isupdated on a frequent basis to calibrate the TRC's, one for each colorof each screen, to produce the final binary page 28, which is printed29.

FIG. 3 shows the halftone step 26 of FIG. 2 in more detail. The printerdependent image 24 is the idealized, neutral, version where, forexample, a midpoint gray is specified as a mid point digital value, soin a system where the range is 0 to 256, 50% gray at this point would bedescribed as a value of 128.

This data is modified by the TRC's at step 30 to produce anarea-coverage version 31 which specifies how many ON pixels are to berequested, and this is the version that is used as the input to thehalftone screening process 32 to produce the binary image raster 28. Itis the generation of these tables, and subsequent modification of thesetables as part of the periodic calibration, that is the subject of thisinvention.

FIG. 4 is a diagram showing the situation in which the effect of thisinvention can be most clearly seen. An edge 40 runs between low 41 andhigh 42 frequency dots. The object is to make the densities as equal aspossible on both sides of the edge. Using this invention, if the lowfrequency dots are the result of a rounding off to make the densitydarker, for example, then using the low frequency TRC value to generatethe high frequency dots will tend to make them darker also, andtherefore to more closely correlate the densities.

While the invention has been described with reference to a specificembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the true spirit and scope of theinvention. In addition, many modifications may be made without departingfrom the essential teachings of the invention.

1. In a printing system, a method for minimizing artifacts produced at aborder produced by printing with a primary halftone screen on one sideof the border and printing with a secondary halftone screen at the otherside of the border, comprising: providing a document to be printed;rendering the document into a full color bit raster; converting therendered document to the printing system's color space; halftoning theconverted document, wherein halftoning include calibration of primaryand secondary halftone screens for each color separation, comprising:generating a first tone reproduction curve (TRC) associated with theprimary halftone screen using input values from a selected colorseparation and primary halftone screen output characteristics, whereineach primary halftone screen output characteristic is a function ofmeasured luminance (delta-e from paper); generating a second TRCassociated with the secondary halftone screen using the first TRC, theprimary halftone screen characteristics and secondary halftone screenoutput characteristics, wherein each secondary halftone screen outputcharacteristic is a function of measured luminance (delta-e from paper),comprising: for each input value in the selected color separation,determining a first TRC output value and an effective primary halftonescreen output characteristic value associated with the first TRC outputvalue; and determining a secondary screen characteristic value havingthe same luminance at the effective primary halftone screen outputcharacteristic value associated with the first TRC output value;determining a second TRC output value based upon the secondary screencharacteristic value having the same luminance as the effective primaryhalftone screen output characteristic value and the input value; andprinting the halftoned document.
 2. The method of claim 1, wherein thesecondary screen frequency is n times the frequency of the primaryscreen frequency and wherein generating a second TRC associated with thesecondary halftone screen comprises: choosing a point P2 of the primaryTRC which corresponds to an input value P1, using point P2 to determineda point P3 corresponding to the primary screen characteristic outputvalue, using point P3 to determine a point P4 corresponding to thesecondary screen characteristic output value having the same luminance,determining a point P5 on the second TRC having an output value that is1/n² of the P1 and the same output value of P4, and rounding off theoutput value of point P5 to the nearest output value integer.
 3. Themethod of claim 1, using a normalized Jones diagram, comprising:choosing a point P2 of the first TRC which has an input value P1, usingpoint P2 to determine a point P3 on the primary screen characteristic,using point P3 to determine a point P4 on the secondary screencharacteristic that has the same luminance, using point P4 to determinea point P5 on the second TRC having the same input value P1, androunding off the output value of point P5 to the nearest output valueinteger.